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Raman spectroscopy of CaMnO 3 : Mode assignment and relationship between Raman line intensities and structural distortions

2002; American Physical Society; Volume: 65; Issue: 18 Linguagem: Inglês

10.1103/physrevb.65.184301

1095-3795

M. V. Abrashev, Joakim Bäckström, L. Börjesson, V. N. Popov, R. A. Chakalov, Nikolay Ivanov Kolev, R.L. Meng, M. N. Iliev,

Solid-state spectroscopy and crystallography

Polarized Raman spectra of oriented finely twinned ${\mathrm{CaMnO}}_{3}$ thin films and nonpolarized Raman spectra of ${\mathrm{CaMnO}}_{3}$ ceramics were studied at room temperature using several excitation laser wavelengths. The selection rules for the polarized Raman spectra, obtained from samples consisting of finely twinned orthorhombic domains, were calculated and the symmetry of all observed Raman lines was determined. The relationship between the intensity of the Raman lines and the distortions in the $\mathrm{AB}{\mathrm{O}}_{3}$ perovskites with ${\mathrm{GdFeO}}_{3}$-type structure is discussed. These distortions can be described as superposition of four simple basic distortions: two ${\mathrm{MnO}}_{6}$ octahedral tilts, Jahn-Teller deformation of ${\mathrm{Mn}}^{3+}{\mathrm{O}}_{6}$ octahedra, and shift of the A ions from their sites in the ideal perovskite. Twenty of the 24 Raman-allowed modes in the real ${\mathrm{GdFeO}}_{3}$-type structure have counterparts in only one of the four simpler structures, obtained by a single basic distortion. The assignment of the Raman lines of ${\mathrm{CaMnO}}_{3}$ to definite atomic vibrations, most of them activated by a single basic distortion, was made in close comparison with the results of lattice dynamical calculations and the Raman spectra of isostructural ${\mathrm{LaMnO}}_{3}$ and ${\mathrm{CaGeO}}_{3}.$